How can we find life off our planet? If we cannot find life itself, can we spot an effect of life, a signature?

This problem famously engaged James Lovelock, he of the Gaia hypothesis, who looked for a signature of life in the distribution of chemicals in the atmosphere of another planet: Mars. He argued that if a planetary atmosphere which was far from chemical equilibrium could only be maintained in that state by some fore of life.

This article, in Technology Review, reports on an effort to find a signature of evolution, which we could possibly use to detect alien biochemistries.